Portable electronic devices can be used for performing a wide variety of tasks, and in some situations the electronic device can be worn on the body of a user. For example, a portable electronic device can be worn by a user on his or her wrist, arm, ankle, or leg. One example of such an electronic device is a wrist-worn activity monitor. The activity monitor can include a heart rate monitor, a position sensor (e.g., gyroscope), and/or a motion sensor (e.g., accelerometer). The activity monitor can determine the type of physical activity based on the signals received from the heart rate monitor and the sensor(s).
Some activities, however, involve little or no limb motion during the performance of the physical activity. For example, a user's arms can remain substantially still when the user is bicycling, walking or running while pushing a stroller, exercising on an elliptical trainer or stair machine while holding the handles or side railings, and performing low-impact activities such as push-ups, squats, or sit-ups. In these situations, it can be difficult, if not impossible, for a wrist-worn activity monitor to determine the type of physical activity the user is performing. The wrist-worn activity monitor may be unable to provide information to the user about the user's physical condition or his or her performance during the physical activity. For example, the wrist-worn activity monitor may not be able to present the user with the number of steps taken by the user or the number of calories expended during the physical activity.